1. Field of the Invention
The present invention relates to a semiconductor laser. More particularly, it relates to an improvement in a semiconductor laser made by a liquid phase epitaxial growth method.
2. Prior Art
Together with remarkable progress of optical communication technology and disk-type video recording apparatus technology, semiconductor lasers are regarded as the most important devices for light sources to be used in such technical fields. Accordingly, there are great requirements in reliability and performance of such semiconductor lasers. That is, lasers of longer lifetime and lasing with a more stable fundamental mode are required more and more.
Various types of semiconductor lasers have been hitherto developed in order to realize lasing operation with a stable fundamental mode. As one of the typical laser structures, a semiconductor laser was proposed in the U.S. patent application of Ser. No. 947,419 (now patented under U.S. Pat. No. 4,188,244 for Itoh et al.). This semiconductor laser is fabricated by the steps of shaping an active layer in a stripe form and growing a high resistive material on both sides of the stripe-shaped active region. The high resistive material has an energy band gap larger and a refractive index smaller than the counterparts of the active region, and therefore the injected current and the lased light are well confined in the active region. The method of forming such a semiconductor laser comprises complicated processing steps such as a growing step of the high resistive material. Besides, there are other shortcomings in such a conventional semiconductor laser (as discussed in the descriptions in the U.S. patent application of Ser. No. 40,182, now U.S. Pat. No. 4,296,387).
Some improvements were proposed in order to eliminate the conventional shortcomings in the semiconductor laser with the buried-in stripe shape active region. One example of such an improved semiconductor laser is shown in FIG. 1. The conventional semiconductor laser of FIG. 1 comprises the following parts:
a substrate 1 of . . . n-GaAs, PA1 a first clad layer 2 of . . . n-Ga.sub.1-x Al.sub.x As, PA1 an active layer 3 of . . . non-doped Ga.sub.1-y Al.sub.y As, PA1 a second clad layer 4 of . . . p-Ga.sub.1-z Al.sub.z As, PA1 a contacting layer 5 of . . . p-GaAs, PA1 an isolation layer 6 of . . . SiO.sub.2, and PA1 electrode contacting layers 7 and 8.
The substrate 1 has a groove 9, on which the double-hetero-structure is formed by crystal growths. It is possible to bury the active layer 3 in the groove 9 by taking advantage of the fact that the crystal growth rate at the bending edges on both sides of the groove 9 is smaller than other flat portions. A lasing operation is obtainable in the active region 3' buried in the groove 9, by providing a stripe-shaped electrode contacting layer 7 for current injection right above the groove 9.
The semiconductor laser of the structure shown in FIG. 1 has the following drawbacks. It is difficult to form the active region 3' in a narrow space between the bending edges on both sides of the groove 9. Further, since the active layer is largely warped at the active region 3' in the groove 9, it is difficult to reproduce fundamental transverse mode lasing well. In addition, it is further difficult to control the crystal growth for the active region 3' with the warped shape. When a crystal growth is carried out in and around the groove 9, one needs to control three different growth rates for the bottom and the sloped portions of the groove 9 and for the flat portions on both sides of the groove 9. Although these crystal growth rates can be varied by changing the supersaturation degree of the epitaxial solution, the crystal growth rates for the three dominant portions must be controlled solely by the supersaturation degree of the epitaxial solution, once the shape of the groove 9 is determined. That is, one certain shape of the active layer 3 is obtained for one certain condition of the supersaturation degree, and the shape of the active layer 3 does not always result in the desired one shown in FIG. 1. Therefore, the warped active region 3' can not be reproduced well.